ORLANDO — After an initial burst of fire between Omar Mateen and a security guard at the Pulse nightclub, a group of five or six police officers arrived on the scene within minutes, broke through a large glass window and entered the club as the killing of 49 people was underway inside, according to a Belle Isle, Fla., police officer who was among the first responders.

Officer Brandon Cornwell, 25, said the ad-hoc team spent the first seconds in the dimly lit club “trying to locate exactly where the shooter was — we kept hearing people scream and shots fired.”

He and the other officers followed the sounds to the bathroom area, where Mateen was now holed up. But instead of entering the bathroom, the officers aimed their assault rifles toward the area and were told by commanders to hold their position as the sounds of gunfire stopped, according to Cornwell. And so they waited “15 or 20 minutes — could’ve been longer” — until the SWAT team arrived, he said. Cornwell never saw Mateen.

Cornwell’s account is the first by a police officer who went inside the club during the first critical moments of the shooting. The FBI said Monday that police first responders “engaged the shooter” inside the club at 2:08 a.m., but Cornwell’s account raises questions about whether gunfire was actually exchanged, why first responders were told not to pursue Mateen into the bathroom, and whether any SWAT or other officers entered the club once the first responders retreated.

Omar Mateen showed no signs of remorse in phone calls with police during an hours-long standoff at the Pulse nightclub, Orlando Police Chief John Mina said in an interview with The Washington Post on June 13. (Gillian Brockell,McKenna Ewen/The Washington Post)

While some survivors described harried rescues by individual officers during the first half-hour or so, others inside the club remained trapped for hours. Some were rescued at 4:21 a.m. — more than two hours after the shooting began — by police working from outside the building. The FBI’s timeline does not describe any SWAT movement into the building until 5 a.m., three hours after the attack started.

“I was yelling, ‘Go in there, go in there, my friends are in there,’ ” said Jeannette McCoy, who escaped the nightclub during the first several minutes and saw the first responders gathering near the main entrance. “People are bleeding to death.”

The Orlando Police Department and the FBI declined to provide further clarification Tuesday. Cornwell also declined to further clarify what happened inside, citing the ongoing investigation and instructions not to talk about such details.

He did not second-guess the decision for the officers to hold their position outside the bathroom.

“We just basically stayed there, waited for movement, and we just held our position until SWAT got there,” said Cornwell, who never fired his weapon. “Once SWAT got there, they told us to retreat, that they’d take over because we were not really in tactical gear — we were just in our police uniforms.”

People who knew Orlando nightclub shooter Omar Mateen describe him as a man who had many demons and potentially led a double life. (Erin Patrick O'Connor,Jayne Orenstein,Thomas LeGro/The Washington Post)

As the FBI continues its investigation of the worst mass shooting in U.S. history, Cornwell and his fellow officers’ early standoff with the shooter — the second of three encounters between law enforcement and Mateen over more than three hours that morning — is being scrutinized by state and federal investigators, along with the other police encounters with the shooter.

Capt. Mark Canty, the Orlando Police Department’s SWAT commander, said that while the incident will be thoroughly reviewed for lessons learned, he believed that everyone “did a good job.”

“That’s the worst part of this. I think we did an outstanding job, but unfortunately people died,” he said.

Chris Cotillo, a former SWAT commander in Prince George’s County, Md., and the current police chief in Seat Pleasant, Md., said that in active shooter situations, officers are now trained to “immediately go in” and “engage the threat.” But in Orlando, he said, the attack presented some unusual quirks. If the shooter stopped firing — and was contained in a bathroom — that would have given officers an opportunity to take stock of the situation, clear out survivors elsewhere in the club and develop a plan.

That Cornwell was even in the vicinity of the Pulse nightclub at 2 a.m. that Sunday was largely a matter of coincidence. The tiny Belle Isle Police Department, situated just south of Orlando in a sleepy community of pale houses and Spanish moss, has an agreement to assist the community of Edgewood, which is near the club. Cornwell, a second-year officer who served with the Army National Guard in Iraq, said he was helping with a traffic stop when he heard the call on his radio that shots had been fired at Pulse. He said he arrived “in 38 seconds.”

Cornwell was in one of the first seven or so police cars to arrive on the scene, where officers were getting out of their vehicles with their assault rifles, he said.

“Some ran towards the building; some stayed back with people running out,” he said. “There was tons of people running out of the club. I grabbed my assault rifle and ran toward the club. At this point, the shooter is still actively shooting inside.”

Cornwell converged on the south side of the building, near the main entrance, with perhaps five other officers, all from the Orlando Police Department, which he referred to as OPD.

“There happens to be an OPD lieutenant commander who was there, and he says, ‘We’ve got to go in,’ ” Cornwell said. “No one disagreed. One of the officers busted out one of those side windows” — it was approximately 10 feet tall — “and we just went in and went from there.”

McCoy, the survivor, who had by then run to the south side of the club as well, described seeing a group of “six to eight” police officers gathered by the entrance with “their guns drawn.” She saw them then shoot through the window, she said.

Cornwell estimated that “no more than two minutes” had elapsed since he and the other officers arrived, and they were now inside the club.

According to the FBI’s timeline, officers “engaged the shooter” inside the club around this time, and three survivors said they heard or saw a brief gun battle.

But Cornwell said Mateen was nowhere to be seen. The club was dim — lit with a disco ball and colored lights — and quiet except for the sound of the shooter’s gunfire, screams and cries for help, Cornwell said.

“He was actively shooting,” he said. “I can’t say if he was targeting us. But he was still shooting in that location where he was at. There were bullet holes in the wall, so he had shot through the wall. But I couldn’t tell you if he was shooting at us.”

Cornwell and the other officers immediately began “clearing rooms” one by one — not knowing if there was more than one shooter — and trying to locate the source of the gunfire. The sound of the shots echoing around the club made it difficult to tell exactly where they were coming from, he said. But fairly quickly — “within minutes,” Cornwell said — officers located Mateen in the bathroom area.

At that point, he said, “we took up a tactical position by the bar standpoint in the middle of the club.” As he aimed his AR-15 assault rifle toward the bathroom door, he said, the shooting stopped. And it was then that the “15 or 20 minute” holding pattern began, he said.

Though Cornwell said he cannot recall exactly how he received his orders — whether via the radio or in person — his clear understanding was that he and his fellow officers were to hold their position rather than attempt to go into the bathroom after the shooter.

Minutes passed as he kept aiming toward the bathroom, he said. He could hear screams. There were people lying all over the floor of the club. He kept aiming, waiting for SWAT. More screaming. He and the other officers held their position, focused on the bathroom, where he could see “some movement inside,” he said.

Asked whether he felt an urge to pursue the shooter at that point, Cornwell said: “I couldn’t tell you. I was following the lieutenant’s command.”

At some point during the 15 to 20 minutes — it is unclear exactly when — Cornwell and the others in the group of first responders exited the club, he said.

“We got word from higher up, and it was communicated to the OPD lieutenant that we needed to withdraw,” he said. “So we came back outside. And waited for SWAT. SWAT arrived. SWAT handled everything from there.”

Multiple survivors have described pauses in Mateen’s gunfire, moments when he left either the main dance floor or the bathroom long enough that some of the survivors were able to place phone calls. What is unclear is whether these movements happened in the first moments, before Cornwell and the other officers entered the club, or after they withdrew.

In the main dance hall, Angel Colon said, he was shot in the first seconds, and then Mateen left the room, only to return and start “shooting everyone who’s on the floor, making sure they’re dead.” Colon told reporters last week that at first he thought the shooter’s absence would give someone else “time to tackle him.”

In the north bathroom, Patience Carter and others were able to make phone calls after Mateen shot them and then appeared to leave the bathroom.

“We laid there for hours and hours . . . hoping that the police would come through at that point in time and just save us all,” she told reporters last week.

Outside, Cornwell said, he spent the next several hours helping transport victims to ambulances. He arrived back at the Belle Isle Police Department on Sunday afternoon, his uniform and all his equipment saturated with blood.